This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
Recent years have seen increased need for an accurate measurement of the water cut—the fraction by volume of water in crude oil relative to the total volume of the mixture. The need has arisen because of the increased use of water and steam for the extraction of crude oil from depleted fields, and because of increased transport of crude by tankers to refineries remote from the field—the transporting tankers often maintain a nominally fixed ballast condition by introducing seawater into oil storage tanks.
Accurate measurement of water cut has proven difficult:                1. Several systems that endeavor to correlate the capacitance of the oil water mixtures with water cut are commercially available. These systems suffer however from several technical difficulties: (a) For high water cuts, the mixture's conductance becomes high and capacitance is a poor measure of the water content; and (b) The relationship between the inter-electrode capacitance (or resistance) may not characterize the true mass or volume fraction of water in the flowing fluid, because the distribution of the phases does not correlate with the electrostatic field intensity.        2. Experiments have shown that, under certain conditions, measurement of the sound velocity of an oil-water mixture can be used to characterize the mass fractions of water and oil in the mixture. There are however several drawbacks to this method: (a) An accurate measure of the sound velocity and specific gravity of each of the two phases is required and (b) The method becomes increasingly inaccurate as the sound velocity and specific gravity of one phase approaches that of the other (this situation can occur with heavy crudes).        3. Many rely on batch sampling of the flowing process fluid for the measurement of water cut, with the separation and weighing of the phases performed off-line. This method has several obvious drawbacks: (a) It is labor intensive, (b) The uncertainties as to how well the sample represents the whole are difficult to bound, and (c) The sample data are not available in real time to allow action in response to a sudden change in water cut.        
The water cut measurement of the present invention draws on the technology of technique 2 above, but overcomes its difficulties, as well as those of the other techniques, by a unique and hitherto unexploited approach.